1. Field of the Invention
The present invention relates to a preparation process of a semiconductor wafer. More particularly, it relates to a preparation process of a semiconductor, which comprises adhering a surface protective adhesive tape having a heat shrinkability on a surface of a semiconductor wafer, such as a silicon wafer, on which an integrated circuit is installed (hereinafter referred to as a front surface of a wafer) grinding another surface of the semiconductor wafer, on which an integrated circuit is not installed (hereinafter referred to as a back surface of a wafer); peeling the adhesive tape from the front surface of the wafer by pouring warm water into the adhesive tape in a wafer cleaning machine; and cleaning the semiconductor wafer in the wafer cleaning machine.
2. Description of the Related Art
A semiconductor integrated circuit (hereinafter referred to as an IC) is generally produced by a process, which comprises slicing a high purity silicon single crystal to make a semiconductor wafer, installing an integrated circuit by a means, such as an etching process, grinding a back surface of the wafer, and dicing to make a chip.
These steps include a step of grinding the back surface of the wafer to thin the thickness of the wafer to an arbitrary thickness after installation of the IC on the front surface of the semiconductor wafer. In this step, an adhesive tape for protecting the front surface of the semiconductor wafer (hereinafter referred to as an adhesive tape) is used by adhering on the front surface of the wafer. After completing the grinding of the back surface of the semiconductor wafer, the adhesive tape is peeled from the front surface of the wafer by a peeling machine. As a method for peeling, a method has been disclosed in JP-A-2-28950, in which a tape having a strong adhesive force called a peeling tape is adhered on the surface of the base film of the adhesive tape, and the adhesive tape is peeled by utilizing the adhesive force.
On the front surface of the semiconductor wafer after peeling of the adhesive tape, a slight amount of an impurity is transferred from the adhesive layer of the adhesive tape. Therefore, the semiconductor wafer is cleaned with an organic solvent, such as isopropyl alcohol and acetone, or pure water in a cleaning machine. In general, pure water is used. There are some methods for cleaning with pure water. For example, a method, in which pure water is poured onto respective single wafers (single wafer process), and a method, in which a cartridge containing about 25 wafers is immersed in a water bath containing pure water overflowing (cartridge process) can be exemplified.
In general, the period of time for grinding the back surface of the semiconductor wafer is about from 100 to 240 minutes per 100 wafers. The period of time for peeling the adhesive tape is about from 140 to 200 minutes per 100 wafers. The period of time for cleaning of the semiconductor wafer carried out after the peeling of the adhesive tape is about from 140 to 600 minutes per 100 wafers.
In recent years, along with the miniaturization of a semiconductor chip, there is a tendency of thinning the wafer. The thickness of the wafer, which has been conventionally about from 200 to 400 .mu.m, becomes as thin as about 150 .mu.m. The size (diameter) of the wafer, which has been 8 inches at most, also tends to be becoming as large as about 12 inches and further 16 inches. According to such a tendency in that the wafer becomes thin and large, the semiconductor wafer sometimes largely warps after grinding the back surface thereof. On the conditions in that an adhesive tape is adhered on the front surface of the semiconductor wafer, such a tendency of warp becomes larger due to the tension of the tape. Therefore, the semiconductor wafer is liable to be broken on peeling the adhesive tape from the front surface of the semiconductor wafer. Furthermore, the semiconductor wafer is also liable to be broken due to vibration on the transfer between the steps, such as transfer from the step of grinding the back surface of the wafer to the step of peeling the adhesive tape, and transfer from the step of peeling the adhesive tape to the step of cleaning the wafer.
In order to prevent the wafer from breakage on peeling the adhesive tape from the front surface of the semiconductor wafer, a surface protective adhesive tape improved in peeling property has been proposed. For example, JP-A-60-189938 discloses a process, in which on grinding the back surface of the semiconductor wafer, an adhesive tape comprising a light transmissible support having thereon a pressure-sensitive adhesive having a property in that it is hardened by light irradiation to form a three-dimensional network is adhered on the front surface of the wafer, and after grinding, a light is irradiated to the adhesive tape, to peel the tape without damaging the wafer.
However, the pressure-sensitive adhesive disclosed in JP-A-60-189938 is an adhesive that is polymerized through radical polymerization, and when oxygen is included between the wafer and the adhesive layer, the hardening reaction cannot sufficiently proceed due to the polymerization inhibition effect of oxygen. Therefore, on peeling the adhesive tape from the front surface of the wafer, an unhardened adhesive having a low cohesive force may remain on the front surface of the wafer, to contaminate the front surface of the wafer. Because an integrated circuit is installed on the front surface of the semiconductor wafer to form complicated unevenness, it is extremely difficult to adhere the adhesive tape without inclusion of the air (oxygen). In order to make a system excluding oxygen to adhere the tape, another apparatus must be newly provided. Such contamination due to the adhesive sometimes can be removed by cleaning with an organic solvent, but in most cases, cannot be completely removed at the present time. In this process, the breakage of the wafer tends to occur on transferring from the peeling step to the cleaning step. Furthermore, it requires the peeling step and the cleaning step, and thus there is no effect on reduction in operation time.
JP-A-8-222535 discloses a method for using an adhesive tape for protection of a front surface of a semiconductor wafer, the tape comprising a stretched film of an ethylene-vinyl acetate copolymer having a shrinking rate at 25.degree. C. of less than 5% and a shrinking rate on immersing in warm water at a temperature of from 50 to 80.degree. C. of from 5 to 50% having on one surface thereof an adhesive layer. The substance thereof resides in a process, in which the adhesive tape is adhered on the front surface of the semiconductor wafer, and after grinding the back surface of the semiconductor wafer, it is immersed in warm water at a temperature of from 50 to 80.degree. C. to peel the adhesive tape from the front surface of the wafer. However, in the method of immersing in warm water, for example, the back surface of the semiconductor wafer cannot be fixed on a chuck table of a peeling machine. Accordingly, it is difficult to protect the semiconductor wafer from the peeling stress of the adhesive tape, and therefore it is impossible to completely prevent the wafer from breakage on peeling the adhesive tape.
In recent years, along with the progress of a semiconductor wafer of large size and small thickness, and an IC of high performance, a preparation process of a semiconductor wafer is demanded, in which contamination of the front surface of the semiconductor wafer is small; the wafer is difficult to be damaged on grinding the back surface of the wafer and peeling the adhesive tape, and the operation time can be reduced.